1. Field of the Invention
This invention relates to an improvement of a pneumatic tire of a type having a tread with a number of grooves which open to the axially outer edges of the tire tread.
2. Related Art Statement
There are known different pneumatic tires of the type having a tread provided with a plurality of grooves each extending diagonally at an angle to the equatorial plane of the tire for improving the wet grip properties of the tire. Such is described in U.S. Pat. No. 4,299,264. Such a pneumatic tire has a plurality of lateral main grooves circumferentially spaced-apart from each other and formed in each side of the equatorial plane or the center line of the tread.
Each lateral main groove is diagonally arranged with respect to the equatorial plane and directed forward forwards in the rotational direction as the main groove approaches the equatorial plane and the axially outermost end of every groove is extended and open to the axially outer edge of the tire tread. As a result, when such a pneumatic tire runs on a wet road, the main grooves serve as passages for draining water between the contacted surfaces of tire tread and the road to improve the wet grip properties of the tire. Such a pneumatic tire is subjected to an axial centrifugal force and is largely deformed when a vehicle turns to the right as shown by the arrow "U" during forward movement of the vehicle and consequently a foot print thereof varies from approximately rectangular shape as shown by the solid line tread portion during a straight drive to substantially a triangular shape as shown by a triangular shape as shown by a chain and dot line in FIG. 20. Thereby a ground contact area i.e. grip area on the turning outside i.e. the outside portion with respect to the equatorial plane with respect to the turning direction of the vehicle remarkably decreases while the ground contact area slightly increases at the inside of equator plane. Thus, the turning performance of the tire substantially depends on the grip area at the outside of equator plane, in particular in the edge portion of the tread when the vehicle turns.
However, in the pneumatic tire of aforementioned type, since the axial outermost end of every lateral groove extends to the tread edge, the rigidity in the tread edge portion decreases larger than that of a conventional tire of rib type and accordingly the ground contact area of the tread remarkably decreases when the tire is subjected to a lateral force. Therefore, the pneumatic tire of aforementioned type has a low cornering performance.